Applicant sought a method for the once-through direct single stage conversion of synthesis or reformer gas to high quality synthetic fuel, to achieve a high yield of hydrocarbons, i.e. a high carbon conversion from CO to hydrocarbons. Synthesis gas has a highly variable H.sub.2 to CO molar ratio of about 2. Synthesis gas from coke or coal generally, has a low H.sub.2 to CO ratio, generally less than about 1.0. Synthesis gas from methane typically has a H.sub.2 /CO ratio greater than 1.9, usually in the range of 2.0-3.0. Reformer gas typically has a H.sub.2 to CO molar ratio of about 3.
Dwyer et al U.S. Pat. No. 4,172,843 discloses a process for single stage catalytic conversion of synthesis gas to hydrocarbon mixtures. This process makes use of two catalysts. The catalysts used are a Fischer-Tropsch catalyst and a methanol conversion catalyst. Dwyer attained a 33% yield using this method.
Fischer-Tropsch catalysts contain one or more metal components from Group VIII of the periodic Table, such as iron, cobalt, nickel, rhodium and ruthenium. Typical Fischer-Tropsch catalysts are potassium promoted fused iron oxide catalysts. These have activity for the conversion of hydrogen and carbon monoxide or carbon dioxide to hydrocarbons, according to the following reactions: EQU CO+2H.sub.2 .fwdarw.(--CH.sub.2 --)+H.sub.2 O EQU CO.sub.2 +3H.sub.2 .fwdarw.(--CH.sub.2 --)+2H.sub.2 O.
However, the product yielded by a Fischer-Tropsch catalyst alone includes a mixture of carbon dioxide, water, alcohols, and hydrocarbons, due to a number of side reactions, more specifically:
the water-gas shift reaction: EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2
the methanol synthesis reaction: EQU CO+2H.sub.2 .revreaction.CH.sub.3 OH
and the formation of methane, by a number of reactions: EQU CO+3H.sub.2 .fwdarw.CH.sub.4 +H.sub.2 O EQU 2CO+2H.sub.2 .fwdarw.CH.sub.4 +CO.sub.2 EQU CO.sub.2 +4H.sub.2 .fwdarw.CH.sub.4 +2H.sub.2 O.
Also, the hydrocarbons produced by a Fischer-Tropsch catalyst alone are of poor quality, containing waxy paraffins which are unsuitable for use in gasoline.
The water-gas shift (WGS) side reaction is of particular significance, since it can lead to high carbon losses, due to the formation of CO.sub.2. This is a reversible exothermic reaction-low temperatures (less than 350.degree. C.) favour forward CO conversion to CO.sub.2, whereas higher temperatures favour the reverse. The water-gas shift reaction changes the oxygen-containing by-products from H.sub.2 O to CO.sub.2, and alters the usage ratio of hydrogen and carbon monoxide in the Fischer-Tropsch synthesis. It has also been shown to occur in parallel with the methanol synthesis reaction starting with CO.sub.2 (Amenomiya, Y. Applied Catalysis, 30, 57-68, 1987).
Dwyer found that if a Fischer-Tropsch catalyst was combined with a methanol conversion catalyst, a better quality product was obtained.
A methanol conversion catalyst, such as crystalline zeolite, has activity for the conversion of methanol into hydrocarbons, according to the following reaction: ##STR1##
Dwyer's combination of a Fischer-Tropsch catalyst and a crystalline zeolite gives a high quality hydrocarbon product. However, there is poor conversion of carbon and a poor yield of 33%. Further, because of the exothermicity of both the Fischer-Tropsch and methanol conversion reactions, Dwyer used a diluent to control and dissipate the heat. The diluent took up half the reaction space and so was not economical.
Another process in the prior art involved combining a methanol conversion catalyst, as described above, with a methanol synthesis catalyst. For example, see Nara, A. et al, Mitsubishi Heavy Industries Ltd. Technical Review Vol. 24 No. 1 (February 1987).
A methanol synthesis catalyst contains one or more metal components with activity for the conversion of hydrogen and carbon monoxide or carbon dioxide into methanol, according to the following reactions: EQU CO+2H.sub.2 .revreaction.CH.sub.3 OH EQU CO.sub.2 +3H.sub.2 .revreaction.CH.sub.3 OH+H.sub.2 O.
Typical examples of a methanol synthesis catalysts include Cu/ZnO/Cr.sub.2 O.sub.3 and Cu/ZnO/Al.sub.2 O.sub.3.
However, methanol synthesis catalysts also are good water-gas shift reaction catalysts.
The combination of methanol synthesis and methanol conversion catalysts produces hydrocarbons according to the following: EQU 2H.sub.2 +CO.revreaction.CH.sub.3 OH.fwdarw.[--CH.sub.2 --]+H.sub.2 O.
However, the useful product yield is low. Large amounts of methane are formed, and carbon is lost to CO.sub.2 due to the water-gas shift reaction.
It should be noted that Dwyer's process of Fischer-Tropsch plus methanol conversion catalysts is favoured by temperatures of 250.degree. to 350.degree. C. Conversely, the combination of methanol synthesis and methanol conversion catalysts is favoured by temperatures greater than 350.degree. C. As these two catalyst combinations are effective at different temperature ranges, it would be expected that they would be incompatible. Further, as the conversion of CO to CO.sub.2 by the water-gas shift reaction as promoted by the methanol synthesis catalyst is favoured at temperatures less than 350.degree. C., it would be expected that if these two systems were combined at the lower temperature, the losses to CO.sub.2 would increase in the presence of the methanol synthesis catalyst. Thus it would be expected that if the two systems were combined, carbon losses would be severe. This result is illustrated by Canadian Patents 1,111,073 and 1,111,074 issued to Shell Canada Limited.
The Shell patents were based on a feedstock from the gasification of coal, which yields a H.sub.2 /CO molar ratio of about 0.5. The claims in Shell were specifically limited to feedstock with H.sub.2 /CO less than 1.
In the Shell patents, conversion of the H.sub.2 and CO to hydrocarbons using the combination of a Fischer-Tropsch catalyst plus a methanol conversion catalyst was desired. However, this combination produces hydrocarbons, as noted above, according to the following reaction: EQU 2H.sub.2 +CO.revreaction.CH.sub.3 OH.fwdarw.[--CH.sub.2 --]+H.sub.2 O.
This requires a H.sub.2 /CO molar ratio of 2. Thus the Shell system was deficient in hydrogen. To overcome this problem a methanol synthesis catalyst was added and the feedstream was modified to include water in order to produce hydrogen according to the water gas shift reaction. In addition to producing the needed H.sub.2, this of course led to very high carbon losses to CO.sub.2. Shell taught utilization of the methanol synthesis catalyst only to the extent necessary to produce H.sub.2 for the hydrocarbon synthesis, as any further production would just increase carbon losses to no purpose. Therefore, it follows from the Shell teachings that a methanol synthesis catalyst should not be used in a system where hydrogen is not deficient.
This point is made by Anderson in his review paper (Anderson, J. R., Methane to Higher Hydrocarbons, Applied Catalysis, 47, p. 183, 1989):
"The formation of H.sub.2 as a primary reaction product means that the possibility exists for the water-gas shift reaction in the reactor. Thus catalysts (e.g. iron) with good activity for the water-gas shift will tend to generate CO.sub.2 rather than water, and simultaneously remove CO and generate H.sub.2. This is a desirable situation when, as with coal-derived synthesis gas, the initial H.sub.2 /CO ratio is low".